The invention disclosed and claimed herein generally pertains to an improved solid state X-ray detector which may be used in computed tomography (CT) diagnostic imaging systems, as well as for other applications. More particularly, the invention pertains to an X-ray detector of the above type which requires an optical coupler material. Even more particularly, the invention pertains to an X-ray detector wherein the optical coupler material is provided with a comparatively low coefficient of thermal expansivity (CTE).
As is well known in the art, solid state X-ray detectors are of increasing importance, for CT imaging and other applications. Such detectors generally include a monolithic block of polycrystalline ceramic scintillator material, which is positioned to receive a flux of X-radiation. The scintillator material may comprise, for example, a material sold by the General Electric Company, assignee herein, under the trademark Lumex, which is proprietary to GE. The X-ray energy received by the scintillator is converted, in proportion to its intensity, to near-visible light as it passes through the scintillator material. The light then excites a photo diode, which is located in close, adjacent relationship with the scintillator, and is designed specifically to convert the light into an electric signal suitable for processing by computer aided means.
In constructing an X-ray detector of the above type, it has been recognized that there must be no air gap or air space between the scintillator and the photo diode. Otherwise, some of the light generated by the scintillator would not be detected by the photo diode, due to light refractivity across the air gap. More particularly, the index of refraction for air is very different from the indices of refraction for the two detector components. As a result, some of the generated light could be reflected by the air-scintillator interface or by the air-photodiode interface, and thereby fail to reach the photodiode. Accordingly, to reduce such refractive losses, it has become common practice to place or sandwich an optical coupler between the scintillator and photodiode. The optical coupler is typically a transparent polymeric material (e.g., an epoxy) introduced as a liquid adhesive that solidifies and bonds the scintillator and photo diode together, and thus prevents any air spaces therebetween.
The above arrangement has generally proved effective in minimizing refractive losses, and thereby enhancing efficiency, in solid state X-ray detectors. However, a scintillator such as Lumex has a coefficient of thermal expansivity or CTE of approximately 6 PPM/xc2x0 C., and a photo diode with a silica glass bonding surface has a CTE of approximately 7 PPM/xc2x0 C. In contrast, an epoxy optical coupler of a type commonly used in the art can have a CTE as high as 200 PPM/xc2x0 C. Such large mismatches in CTE can cause thermally induced strains at the optical coupler-scintillator and optical coupler-photodiode interfaces, which in turn may result in optical coupler detachment when the X-ray detector is exposed to temperature extremes. That is, as the X-ray detector experiences a large temperature change, dimensions of the optical coupler are likely to change by a much greater extent than dimensions of the scintillator and photo diode. Accordingly, the optical coupler may be subjected to substantial stress, and may even crack or become separated from other X-ray detector components.
The invention disclosed and claimed herein is directed to a comparatively simple and inexpensive technique for substantially reducing the coefficient of thermal expansivity of an optical coupler for a solid state X-ray detector, in comparison with the prior art. In one mode, the invention provides an X-ray detector apparatus which includes a first quantity of selected ceramic scintillating material, formed to comprise a monolithic scintillator body having a specified face. The scintillator body is disposed to receive an amount of X-radiation, and to project an amount of light corresponding thereto through the specified scintillator face. The detector apparatus further includes a photodiode device spaced apart from the specified scintillator face by a gap of selected width, the photodiode device being disposed to receive the projected light and to produce an electric signal which is proportional thereto, and is thereby proportional to or otherwise represent the amount of X-radiation received by the scintillator body. An optical coupling material of selected viscosity is positioned in the gap to reduce refractivity of light traversing the gap. A second quantity of the selected scintillating material, which is in the form of a powder, is mixed with the optical coupling material to provide an optical coupler composition of selectively reduced thermal expansivity.
Preferably, the scintillating material comprises a polycrystalline ceramic material such as Lumex which includes specified amounts of gadolinium oxide and a rare earth activator. In a preferred embodiment, the optical coupling material comprises a transparent epoxy material, and the X-ray detector is adapted for use in a CT imaging system. It is anticipated that an embodiment of the invention can provide an improved optical coupler for an X-ray detector having a CTE which is significantly lower than the CTE of an optical coupler conventionally used in solid state X-ray detectors.
In another mode, the invention is directed to a method for constructing an X-ray detector assembly for a CT imaging system. The method comprises mixing a prespecified amount of a ceramic scintillator material, in powdered form, with the resin component of an optical coupling material having resin and hardener components. Thereafter, air bubbles are selectively removed from the powdered scintillator-resin mixture. The method further comprises mixing a prespecified amount of the hardener component of the optical coupling material with the powdered scintillator-resin mixture to provide a powdered scintillator-optical coupler composite, and then removing air bubbles from the composite. A monolithic block or body of the ceramic scintillator material is placed in close, spaced-apart relationship with a photodiode device, to provide a gap of specified width therebetween, and the gap is filled with the composite to eliminate air spaces therefrom.